Field of the Invention
This invention relates to image forming apparatuses such as printers, copy machines, printing machines, facsimile machines, etc.
Image forming apparatuses such as various printers, copy machines, printing machines, etc., use removable paper feed cassettes or trays in their paper feed units.
Many use a friction pad type paper feed units to feed, one by one, sheets of papers stacked in the trays. In the paper feed unit, many stacked sheets of papers are pushed against a feed roller by means of a bottom plate with a friction pad, and the uppermost sheet of paper is fed to a predetermined position where the sheet of paper contacts conveyor rollers (resist roller) by rotating the feed roller, then the driving of the feed roller is stopped, thereby rendering the feed roller freely rotatable.
Thereafter, the conveyor rollers are started with a predetermined timing and carries the sheet of paper to an image forming unit for example, a toner image transfer unit located by a photosensitive drum.
The feed roller in such feed unit has an outer periphery made of a material having a large coefficient of friction such as rubber and the friction pad is also made of a material having a large coefficient of friction such as cork or rubber.
Only the uppermost sheet of paper, contacting the feed roller, is arranged to be fed out one by one under such a condition that EQU .mu..sub.1 &gt;.mu..sub.2 &gt;.mu..sub.3
where .mu..sub.1 is the coefficient of friction between the feed roller and a sheet of paper, .mu..sub.2 the coefficient of friction between the friction pad and a sheet of paper, and .mu..sub.3 the coefficient of friction between sheets of paper.
When the last sheet of paper is to be fed in such a friction pad type feed unit, the sheet of paper is directly pushed against the feed roller by the friction pad. If the feed roller stops and the trailing end of the sheet of paper is not out of the bite of the feed roller and friction pad, the sheet of paper is in a state where the sheet of paper is held on both sides thereof with large frictional force.
When the sheet of paper starts to be fed out of the bite of the feed roller and friction pad, the friction pad starts to push the feed roller directly, and an increasing braking force starts to act on the feed roller. Thus the force to feed the sheet of paper increases rapidly. Therefore, if the conveyance force caused by conveyor rollers is not greater than the braking force, the sheet of paper cannot be conveyed, knocking occurs so that smooth conveyance is not expected, and local image shifts or torsions will be caused in a reproduced image.
If paper jamming occurs in each of the image forming apparatuses such as various printers, copy machines or printing machines for some reason during the formation of an image, the apparatus immediately discharges a dischargeable sheet of paper, stops its operation, and waits until the jammed sheet of paper is removed and a jam release switch is pressed. The operator then opens the cover or removes the cassette and takes out the jammed sheet of paper by pinching it with one's fingers. It is time-consuming to remove all the jammed sheets of paper present in the paper feed path in the image forming apparatus.
In an image forming apparatus in which a sheet of paper on which an image will be formed is fed out from a cassette or a reversing tray provided at the bottom of the apparatus proper and/or the sheet of paper on which an image is formed is discharged to a discharge tray provided on the top of the apparatus, a paper conveyance unit is provided which conveys a sheet of paper substantially in a U-like bent manner. If a jammed sheet of paper is present at that paper conveyance unit, it is usually bent close to the feed roller or a guide plate, so that it is difficult to pinch away the sheet of paper with fingers, and it takes much time to remove the sheet of paper.